1. Field of the Invention
The present invention relates to an imaging apparatus and an exposure control method of the same and, particularly, to an imaging apparatus including an image pickup device to generate a plurality of captured images and an exposure control method of the same.
2. Description of the Related Art
For imaging apparatus including an image pickup device such as a CCD (Charge Coupled Device), various techniques have been proposed to reduce noise and increase dynamic range in a quest to improve the quality of captured images.
A common technique to increase dynamic range is to combine a plurality of images by overlaying one another in a CCD sensor. This technique corrects image blurring (displacement) due to hand-shake or the like when superimposing a plurality of images on one another to thereby increase dynamic range and suppress deterioration of image quality.
Typically, a CCD sensor accumulates charges in a photoreceptor through exposure, discharges (sweeps out) the accumulated charges to a substrate, sequentially transfers out the discharged charges, and then outputs a captured image. A CCD sensor carries out the exposure, charge discharging and image output in response to control signals supplied from a controller. An exposure (shutter) time of a CCD sensor is controlled by a SUB pulse (charge discharging pulse), which is one of control signals supplied from a controller to a CCD sensor.
There is some kind of noises occurring in a CCD sensor, which causes deterioration of image quality. A dark charge (dark current) noise is one kind of noises that occur in a CCD sensor. The dark charge noise is caused by charges that are accumulated irrespective of light in the light blocked condition. The dark charge noise is generated mostly by SUB pulses that are applied to a CCD sensor.
FIG. 5 is a conceptual illustration of a captured image where a dark charge noise occurs. A captured image is formed by a CCD sensor that outputs pixels (charges) row by row from the top to the bottom of the image. In FIG. 5, the reference numeral 501 indicates an image of a non-exposure period when a SUB pulse is supplied during image output and exposure is not performed, and 502 indicates an image of an exposure period where a SUB pulse is stopped during image output from a CCD sensor and exposure on the next image is performed. A dark charge noise 503 is generated horizontally on the boundary between the non-exposure period 501 and the exposure period 502.
Therefore, for still shoot in which high quality image is required, an image during the non-exposure period (during the supply of SUB pulses) is often discarded without being used. However, the image blurring correction through image superimposition requires a plurality of still-shot images that are continuously taken at the shortest possible time intervals and it is thus necessary to carry out the exposure and the image output at the same time.
FIG. 6 illustrates that a plurality of captured images are superimposed on one another in an imaging apparatus of a related art. When correcting image blurring by way of image superimposition, the imaging apparatus of a related art takes a plurality of images at an exposure time per captured image (B/N) that is calculated by dividing an appropriate exposure (total exposure time B per composite image) by the number N of images taken and then superimposes the N number of captured images on one another. In this example, three captured images from 601a to 601c are superimposed on one another to thereby produce a composite image 602 after correction.
As shown in FIG. 6, a CCD sensor performs exposure and image output at an interval of Vsync (vertical synchronization) signal. A period during which a SUB pulse is supplied is a non-exposure period, a period during which a SUB pulse is not supplied is an exposure period, and period during which the charges discharged by a SUB pulse are output is an image output period.
In the captured images from 601a to 601c, the exposure starts during the image output period, and a dark charge noise occurs at the position of the start of the exposure. In the imaging apparatus of a related art, the non-exposure period when a SUB pulse is applied and the exposure period when a SUB pulse is not applied are the same for each captured image and thus the timing to start the exposure is the same, which causes a dark charge noise to occur at the same position in every captured image. As a result, in the composite image 602 that is produced by superimposing such captured images, the dark charge noises of the captured images overlap to enhance the noise to appear remarkably, which significantly deteriorates the image quality.
Japanese Unexamined Patent Application Publication Nos. 2002-10143 (Yoshida et al.) and 2001-268449 (Sakurai et al.) disclose imaging apparatus to reduce such dark charge noises.
The imaging apparatus of related arts such as Yoshida et al. and Sakurai et al. suppress the deterioration of image quality due to noise by way of controlling exposure by a timing to open and close a physical shutter (mechanical shutter) and a timing of SUB pulses. Therefore, the imaging apparatus of related arts cannot reduce noises such as a dark charge noise to suppress the deterioration of image quality unless it has a physical shutter.